This invention relates generally to a bipolar signal apparatus and method and more particularly, but not by way of limitation, to a system and method of transmitting a very low frequency signal on a drill string disposed in a well bore.
Amplification or generation of very low (and variable) frequency, low voltage, high current sine-wave signals is used in electrical borehole telemetry (EBT) systems and other applications. In EBT systems, for example, a transmitter connected to a drill string and lowered into a well bore transmits a very low frequency signal through the drill string to a receiver at the surface. The signal is encoded with information such as from pressure or temperature sensors also located within the well bore. The current loop of the signal propagation includes the drill string and the earth as indicated by the current representations i.sub.1, i.sub.2, i.sub.3 in FIG. 1. Such very low frequency signals can also be transmitted from the surface to control something downhole.
In EBT, a voltage is typically impressed across an insulating gap such that a current is caused to flow in the conductor as shown in FIG. 1. Because the transmission path in the earth is a very low impedance, a low voltage, high current signal is required for successful operation. It is also known that a very low frequency sine-wave signal is well suited to such telemetry. An important feature of such a sine-wave signal is that it be symmetrical about zero potential such that no power is lost due to direct current flow in the conductor. If the telemetered signal is to convey information, it must also be capable of being appropriately modulated.
One type of downhole telemetry apparatus is disclosed in U.S. Pat. No. 4,691,203 to Rubin et al. The Rubin et al. apparatus uses pulse width modulation in converting an input signal to an output signal for transmission from a downhole drill string location. A high frequency impedance matching transformer is used in the conversion technique. It would be preferable not to have to use a transformer so that cost and space requirements could be reduced.
The circuit shown in FIG. 2 is a step-down switching type voltage regulator circuit that is commonly used to transform a positive dc power source to a lower positive dc output voltage. A similar circuit configured for negative voltages is illustrated in FIG. 3. In the circuits of both FIGS. 2 and 3, the power source voltage is periodically connected to a load R.sub.L through a gate device and an inductor-capacitor integrating network in response to a gate on-off control signal provided by a duty cycle control circuit. Although the gate device is illustrated as a bipolar transistor, any suitable switching device can be used. When the circuits of FIGS. 2 and 3 are configured to deliver a high current signal to the load, a number of such gate devices may be combined in parallel to share the load current and reduce ohmic related gate power loss. The gate control signal is typically a series of rectangular pulses in time which are controlled to represent a variable on-off duty cycle.
The duty cycle is normally related to the difference of the output voltage and a stable selected reference voltage. When the gate is on, current pulses are delivered to the inductor. When the gate is turned off, the cessation of current flow in the inductor drives its input side to an opposite polarity voltage relative to the output voltage. The purpose of the diode in the circuits of both FIGS. 2 and 3 is to clamp the inductor input to ground and thereby to impress the inductor voltage on the load. When the gate is off, therefore, current is delivered to the load by the inductor from the energy stored in its field during a prior period when the gate was on. A more detailed analysis of the operation of the basic circuit indicates that the output voltage-to-power source voltage is proportional to the gate duty cycle, and that no fundamental circuit efficiency limitation is inherent in its operation.
The linear gate duty cycle-to-output voltage relationship, inherently high circuit power efficiency and minimal quantity of components are features of the step-down switching regulators of FIGS. 2 and 3 that are desirable in the amplification or generation of an EBT signal. In particular, the absence of the requirement for a signal voltage level converting transformer is very desirable. Such transformers, suitable for use with the low frequency signals that are required for proper operation of an EBT system, are typically very large. The Rubin et al. apparatus was trying to get away from this very large type of transformer by using a high frequency transformer; however, even such a high frequency transformer requires space which is at a premium in a downhole location.
Although the circuits shown in FIGS. 2 and 3 have desirable characteristics, they produce dc outputs, and an ac output is required in an EBT system. Thus, there is the need for a non-transformer type of bipolar signal system which in a preferred embodiment is capable of producing very low frequency signals for transmission on a drill string.